diff --git a/configs/tsconfig-mongosh/package.json b/configs/tsconfig-mongosh/package.json
index 10ea57aeb..4b3440b2b 100644
--- a/configs/tsconfig-mongosh/package.json
+++ b/configs/tsconfig-mongosh/package.json
@@ -5,8 +5,7 @@
   "description": "Shared Mongosh Typescript configuration",
   "license": "SSPL",
   "files": [
-    "tsconfig.common.json",
-    "tsconfig.test.json"
+    "tsconfig.common.json"
   ],
   "peerDependencies": {
     "typescript": "^5.3.3"
diff --git a/configs/tsconfig-mongosh/tsconfig.test.json b/configs/tsconfig-mongosh/tsconfig.test.json
deleted file mode 100644
index 0045835f9..000000000
--- a/configs/tsconfig-mongosh/tsconfig.test.json
+++ /dev/null
@@ -1,13 +0,0 @@
-{
-  "compilerOptions": {
-    "esModuleInterop": true,
-    "downlevelIteration": true,
-    "strict": false,
-    "noUnusedLocals": false,
-    "noUnusedParameters": false,
-    "target": "es2018",
-    "lib": ["es2019"],
-    "module": "commonjs",
-    "moduleResolution": "node"
-  }
-}
diff --git a/packages/node-runtime-worker-thread/package.json b/packages/node-runtime-worker-thread/package.json
index 676522539..23863a126 100644
--- a/packages/node-runtime-worker-thread/package.json
+++ b/packages/node-runtime-worker-thread/package.json
@@ -17,7 +17,7 @@
     "node": ">=14.15.1"
   },
   "scripts": {
-    "test": "cross-env TS_NODE_PROJECT=./tsconfig.test.json mocha -r \"../../scripts/import-expansions.js\" -r \"./tests/register-worker.js \" --timeout 15000 -r ts-node/register \"./src/**/*.spec.ts\"",
+    "test": "mocha -r \"../../scripts/import-expansions.js\" -r \"./tests/register-worker.js \" --timeout 15000 -r ts-node/register \"./src/**/*.spec.ts\"",
     "pretest-ci": "node ../../scripts/run-if-package-requested.js npm run webpack-build -- --no-stats --no-devtool",
     "test-ci": "node ../../scripts/run-if-package-requested.js npm test",
     "test-coverage": "nyc --no-clean --cwd ../.. --reporter=none npm run test",
diff --git a/packages/node-runtime-worker-thread/src/index.spec.ts b/packages/node-runtime-worker-thread/src/index.spec.ts
index bd0fd7446..a1aa5e909 100644
--- a/packages/node-runtime-worker-thread/src/index.spec.ts
+++ b/packages/node-runtime-worker-thread/src/index.spec.ts
@@ -48,7 +48,7 @@ describe('WorkerRuntime', function () {
           nodb: true,
         });
 
-        let err: Error;
+        let err!: Error;
 
         try {
           await runtime.evaluate('throw new TypeError("Oh no, types!")');
@@ -219,7 +219,7 @@ describe('WorkerRuntime', function () {
       const runtime = new WorkerRuntime('mongodb://nodb/', dummyOptions, {
         nodb: true,
       });
-      let err: Error;
+      let err!: Error;
       try {
         await Promise.all([
           runtime.evaluate('while(true){}'),
@@ -246,7 +246,7 @@ describe('WorkerRuntime', function () {
 
         await runtime.waitForRuntimeToBeReady();
 
-        let err: Error;
+        let err!: Error;
 
         try {
           await Promise.all([
@@ -329,7 +329,7 @@ describe('WorkerRuntime', function () {
 
         await runtime.waitForRuntimeToBeReady();
 
-        let err: Error;
+        let err!: Error;
 
         try {
           await Promise.all([
diff --git a/packages/node-runtime-worker-thread/src/lock.spec.ts b/packages/node-runtime-worker-thread/src/lock.spec.ts
index 35ead53df..dde21e9f8 100644
--- a/packages/node-runtime-worker-thread/src/lock.spec.ts
+++ b/packages/node-runtime-worker-thread/src/lock.spec.ts
@@ -25,7 +25,7 @@ describe('Lock', function () {
     // eslint-disable-next-line @typescript-eslint/no-floating-promises
     lock.lock();
 
-    let err: Error;
+    let err!: Error;
 
     try {
       // eslint-disable-next-line @typescript-eslint/no-floating-promises
diff --git a/packages/node-runtime-worker-thread/src/rpc.spec.ts b/packages/node-runtime-worker-thread/src/rpc.spec.ts
index ac7880299..e94e765b1 100644
--- a/packages/node-runtime-worker-thread/src/rpc.spec.ts
+++ b/packages/node-runtime-worker-thread/src/rpc.spec.ts
@@ -27,7 +27,7 @@ describe('rpc', function () {
     woof(...args: any[]): string;
     neverResolves(...args: any[]): void;
   }>;
-  let exposed: Exposed<unknown> | null;
+  let exposed: Exposed<unknown>; // adding `| null` breaks TS type inference
 
   afterEach(function () {
     if (messageBus) {
@@ -36,12 +36,12 @@ describe('rpc', function () {
 
     if (caller) {
       caller[cancel]();
-      caller = null;
+      caller = null as any;
     }
 
     if (exposed) {
       exposed[close]();
-      exposed = null;
+      exposed = null as any;
     }
   });
 
@@ -74,7 +74,7 @@ describe('rpc', function () {
       messageBus
     );
 
-    let err: Error;
+    let err!: Error;
 
     try {
       // eslint-disable-next-line @typescript-eslint/await-thenable
@@ -136,7 +136,7 @@ describe('rpc', function () {
       messageBus = createMockRpcMesageBus();
       caller = createCaller(['meow'], messageBus);
 
-      messageBus.addEventListener('message', (data) => {
+      messageBus.addEventListener('message', (data: unknown) => {
         expect(data).to.have.property('func', 'meow');
         done();
       });
@@ -150,7 +150,7 @@ describe('rpc', function () {
       it('stops all in-flight evaluations', async function () {
         messageBus = createMockRpcMesageBus();
         caller = createCaller(['neverResolves'], messageBus);
-        let err: Error;
+        let err!: Error;
         try {
           await Promise.all([
             caller.neverResolves(),
diff --git a/packages/node-runtime-worker-thread/src/rpc.ts b/packages/node-runtime-worker-thread/src/rpc.ts
index 53bf50a56..3672c9fd8 100644
--- a/packages/node-runtime-worker-thread/src/rpc.ts
+++ b/packages/node-runtime-worker-thread/src/rpc.ts
@@ -50,7 +50,10 @@ export type Exposed<T> = { [k in keyof T]: T[k] & { close(): void } } & {
   [close]: () => void;
 };
 
-export function exposeAll<O>(obj: O, messageBus: RPCMessageBus): Exposed<O> {
+export function exposeAll<O extends object>(
+  obj: O,
+  messageBus: RPCMessageBus
+): Exposed<O> {
   Object.entries(obj as Record<string, any>).forEach(([key, val]) => {
     const { close } = expose(
       key,
diff --git a/packages/node-runtime-worker-thread/src/worker-runtime.spec.ts b/packages/node-runtime-worker-thread/src/worker-runtime.spec.ts
index 41dc7a361..b10991482 100644
--- a/packages/node-runtime-worker-thread/src/worker-runtime.spec.ts
+++ b/packages/node-runtime-worker-thread/src/worker-runtime.spec.ts
@@ -63,14 +63,14 @@ describe('worker-runtime', function () {
 
     if (caller) {
       caller[cancel]();
-      caller = null;
+      caller = null as any;
     }
   });
 
   it('should throw if worker is not initialized yet', async function () {
     const { evaluate } = caller;
 
-    let err: Error;
+    let err!: Error;
 
     try {
       await evaluate('1 + 1');
@@ -161,18 +161,18 @@ describe('worker-runtime', function () {
     describe('shell-api results', function () {
       const testServer = startSharedTestServer();
       const db = `test-db-${Date.now().toString(16)}`;
-      let exposed: Exposed<unknown> | null;
+      let exposed: Exposed<unknown>; // adding `| null` breaks TS type inference
 
       afterEach(function () {
         if (exposed) {
           exposed[close]();
-          exposed = null;
+          exposed = null as any;
         }
       });
 
       type CommandTestRecord =
         | [string | string[], string]
-        | [string | string[], string, any];
+        | [string | string[], string | null, any];
 
       const showCommand: CommandTestRecord[] = [
         [
@@ -323,11 +323,11 @@ describe('worker-runtime', function () {
         .forEach((testCase) => {
           const [commands, resultType, printable] = testCase;
 
-          let command: string | undefined;
+          let command: string;
           let prepare: undefined | string[];
 
           if (Array.isArray(commands)) {
-            command = commands.pop();
+            command = commands.pop()!;
             prepare = commands;
           } else {
             command = commands;
@@ -376,7 +376,7 @@ describe('worker-runtime', function () {
 
         await init('mongodb://nodb/', dummyOptions, { nodb: true });
 
-        let err: Error;
+        let err!: Error;
         try {
           await evaluate('throw new TypeError("Oh no, types!")');
         } catch (e: any) {
@@ -396,7 +396,7 @@ describe('worker-runtime', function () {
 
         await init('mongodb://nodb/', dummyOptions, { nodb: true });
 
-        let err: Error;
+        let err!: Error;
         try {
           await evaluate(
             'throw Object.assign(new TypeError("Oh no, types!"), { errInfo: { message: "wrong type :S" } })'
@@ -430,7 +430,7 @@ describe('worker-runtime', function () {
         const { init, evaluate } = caller;
         await init('mongodb://nodb/', dummyOptions, { nodb: true });
 
-        let err: Error;
+        let err!: Error;
 
         try {
           await Promise.all([
@@ -665,7 +665,7 @@ describe('worker-runtime', function () {
 
         await init('mongodb://nodb/', dummyOptions, { nodb: true });
 
-        let err: Error;
+        let err!: Error;
 
         try {
           await Promise.all([
@@ -695,7 +695,7 @@ describe('worker-runtime', function () {
 
       await init('mongodb://nodb/', dummyOptions, { nodb: true });
 
-      let err: Error;
+      let err!: Error;
 
       try {
         await Promise.all([
diff --git a/packages/node-runtime-worker-thread/tsconfig.test.json b/packages/node-runtime-worker-thread/tsconfig.test.json
deleted file mode 100644
index 3676cf085..000000000
--- a/packages/node-runtime-worker-thread/tsconfig.test.json
+++ /dev/null
@@ -1,11 +0,0 @@
-{
-  "extends": "@mongodb-js/tsconfig-mongosh/tsconfig.test.json",
-  "ts-node": {
-    "files": true
-  },
-  "compilerOptions": {
-    "outDir": "./lib",
-    "allowJs": false
-  },
-  "files": ["./src/index.d.ts"]
-}